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1. Field of Invention
This invention pertains to recording and reading information on magnetic tape, and particularly to error techniques for handling re-recording and/or reading a frame of information when it has been discovered that the frame was originally erroneously recorded or is unreadable from the tape.
2. Related Art and Other Considerations
In magnetic tape recording, a head unit having one or more head elements transduces information onto or from an elongated magnetic tape which is transported past the head unit. Information is typically recorded in units known as blocks or frames on tracks or stripes of the magnetic tape. The tracks or stripes are formed in dependence upon the transport direction of the tape and (in cases of helical scan recording, for example) any relative motion of the head unit relative to the tape. In view of factors including the transport of the tape, magnetic tape recording is conventionally viewed as a sequential recording technique, since the blocks or frames must be accessed one at a time and in order in a tape read process.
Many magnetic tape recording devices, sometimes known as drives or recorders, have the capability of reading back a block or frame soon after it is recorded in order to determine the fidelity of the recorded block or frame. This capability is generally referred to as "read after write" or "read while write", and is typically implemented by providing the head unit with a read element which follows a write element as it writes a track. If a block or frame as recorded as read back does not agree with what the drive intended to record or otherwise does not meet prescribed error correction criteria, the recorded block or frame is determined to be a bad block or bad frame.
Upon discovery of a defective frame, magnetic tape drives conventionally attempt to rewrite the frame in the next available physical location on the tape. Some techniques essentially involve the re-recording of frames which follow the re-recorded frame, since other frames may have been recorded before the drive detected the discovery of the recordation of the bad frame. See, for example, U.S. Patent U.S. Pat. No. 4,637,023 to Lounsbury et al.
Another technique is the re-recording of a frame or block in a subsequent user-available location on the tape. That is, the frame again appears in the output stream of the drive and is re-recorded in the course of recording subsequent blocks in a non-restricted, user-available portion of the tape. The subsequent location may be chosen to avoid potential media defects or may depend upon selection of one of a plurality of heads in the head unit. See U.S. Pat. No. 5,142,422 to Zook et al., which is incorporated herein by reference.
Magnetic disks have differing operation constraints and conditions than magnetic tape. Conventional practice in the magnetic disk technology is to reserve a portion of the disk for re-recording of bad sectors, the reserved portion of the disk being unavailable for access by the user's other data. The magnetic disk typically has a map of bad sectors, so that a disk drive knows to locate a sector in its bad sector map in the reserved portion of the disk. See, for example, U.S. Pat. No. 4,638,472 to Ogata et al.
Some magnetic tapes are specially prepared by the tape manufacturer to have a servo signal or the like permanently recorded thereon at a frequency which is outside the frequency range of the data which will be recorded on the tape by the user's tape drive. Such a servo signal, e.g, a buried servo signal, can serve to form demarcations or sectoring of the tape, known as hard sectors, such that the blocks or frames recorded on the tape each have a permanent physical location. In a hard sectored format, a tape may be used as a random access device allowing the host to select the location. For the host to be able to select the best locations, the partition geometry relative to the data locations must not be corrupted.
When using such a hard sectored tape with fixed physical locations for frames, and assuming the drive has read while write capabilities, there is a problem when one records or updates a frame and then discovers, during the read back of the frame, that the frame was erroneously recorded or is not readable from the tape. Standard techniques of rewriting the frame on tape at subsequent frame position on the tape cannot be employed, since that subsequent frame position has a fixed physical location which has already been allocated to another frame of the tape.
What is needed therefore, and an object of the present invention, is a technique for handling the re- recording and reading of frames which were erroneously recorded on hard sectored magnetic tape.